Start at the iliac crest (the top of your hipbone that you can feel like a shelf on the side of your body). From this level on your spine, measure to the C7 vertebrae, the knobby bone at the base of your neck when you put your head down. Make sure to keep your back straight while having the measurement taken.

Awesome pack for long distance

Gender:
Male

Familiarity:
I've put it through the wringer

Used this pack the entire length of the AT in 2012 and loved it the whole way. I have 2 other Osprey packs (Argon 85 and Aether 60) and this one fits the best. I have the previous model, but several of my firends had this newer version and not much has changed. In fact, I will say that about 80% of the hiker class from 2012 had some model of Osprey pack. The Atmos and Exos were the most popular, but did see a few Talon 44's and Kestrel models. The girl I hiked with had a Gregory and wished she'd bought an Osprey after hearing such rave reviews by fellow hikers. 2184.2 miles equals a well tested product and high approval rating! If you are planning to thru-hikeor just looking for a great pack for short trips, do yourself a favor and buy this pack!

In addition, I'm 5'9" and have a 31" waist. I bought a Medium and was a perfect fit.